For many years it has been common practice to couple two ends of corrugated metal pipe by wrapping the ends with some form of sheet metal band that is drawn up or tightened by means of bolts. This type of coupling has been used both with regard to circular and spiral formed corrugated metal pipe. With spiral formed pipe, however, the practice may include the step of reforming the pipe ends with angular reforming dies.
Within the past few years, a new pipe product, known as box rib or spiral ribbed pipe, has been developed. Such pipe constructions are reasonably smooth in the bore as compared with the standard corrugated metal pipe and consequently have less resistance to fluid flows. The couplings used for box rib or spiral ribbed pipe are largely those commonly employed for pipe having annular corrugations. This necessitates that the ends of the box ribbed pipe be reformed into an annular configuration, just as the spirally corrugated pipe has been reformed. But the box rib or spiral rib pipe does not reform well. The size and the amount of metal that exists in the box rib portion of the pipe creates distortions. Reforming is not only expensive but difficult to accomplish to provide a watertight coupling between ends of two pipe sections.
Some manufacturers have avoided the problem of reforming by welding a flanged collar onto the ends of the pipe sections, then connecting the flanged portions of each flanged collar together by means of bolts or bands. Until now that practice has been the only practical method for coupling box rib or spiral rib metal pipe, particularly where a watertight joint is specified. But it is also recognized in the pipe industry that the cost of welding flanges to galvanized steel and aluminum pipes is very high, and it requires a great deal of patience and talent to accomplish an acceptable weld.